Semiconductor devices ordinarily require passivating coatings particularly in the vicinity of junctions. Such passivation has a three-fold effect. It acts to terminate the crystal lattice in a way that preserves the desired electrical characteristics, it reduces the effect of ionic contaminants at the ambient-device interface, and it provides mechanical protection for the semiconductor surface. Typically such a layer acts upon the surface in a way that does not alter surface recombination, at least it does not increase recombination excessively. It must not act as a donor or acceptor material and should not introduce mechanical strain. Particularly in the case where relatively thick coatings are desired, thermal expansion mismatch should be avoided. This is of paramount importance when dealing with devices that operate at cryogenic temperatures and therefore are subject to cycling over very large temperature ranges.
Silicon dioxide has proven to be an excellent material for the passivation of silicon devices. More recently it has been found that if the silicon dioxide is overlaid with an oxide containing phosphorous oxide, even better passivation occurs. In general, glass and simple mixtures of metal oxides and/or nitrides have been used for passivation, and in certain special cases organic compounds have proven effective. In the case of silicon, the genetic oxide, or one that is grown from the parent silicon crystal, has proven to be the most compatible. Typically the genetic oxide is limited in thickness and is overlaid with a deposited oxide or oxide mixture.
In the case of germanium satisfactory passivation is not as simple or easy as for silicon. The genetic oxide is not usually acceptable mechanically, it is a poor insulator, it volatalizes at the elevated temperatures that are normally found in device processing, and is easily dissolved by many solvents normally used in device processing. Accordingly germanium passivation is difficult and many different processes have been developed. Commonly that passivation process is employed having the fewest deleterious effects on device performance while providing the required mechanical properties. For example, silicon dioxide deposited onto a clean germanium surface will provide some passivation but it is not good either electrically or mechanically.